Hydrothermal Synthesis of Nanosized CoAl2O4 on ZnAl2O4 Seed Crystallites

ZnAl₂O₄-seeded CoAl₂O₄, with a core-shell structure, has been prepared under hydrothermal conditions when the Co²⁺ salt solution is substituted by 10% Zn²⁺ as a precursor. The ZnAl₂O₄ seed is generated during the synthesis process. The seeding process can decrease the synthesis temperature from 245° to 230°C and the particle size from 67 to 20 nm. The process can economize the consumption of Co²⁺ and control the particle size effectively.     

Formation of NiAl2O4 by Solid State Reaction

The growth of nickel-aluminum spinel, NiAl₂O₄, in diffusion couples of polycrystalline Al₂O₃ and NiO was investigated between 1200° and 1500°C. The growth kinetics for the spinel layer obeyed a parabolic rate law in this temperature range. Marker experiments showed that the spinel layer formed by counterdiffusion of nickel and aluminum ions. Comparison of experimental and theoretical values of the parabolic rate constants suggests that the diffusion of aluminum ions through the spinel layer is rate controlling.     

Activity–Composition Relations in NiAl2O4─MnAl2O4 Solid Solutions and Stabilities of NiAl2O4 and MnAl2O4 at 1300° and 1400°C

Activities of NiO were measured in the oxide and spinel solutions of the system MnO–NiO–Al₂O₃ at 1300° and 1400° C with the aim of deriving information on the thermodynamic properties of the spinel phases. Synthetic samples in selected phase assemblages of the system were equilibrated with metallic nickel and a gas phase of known oxygen partial pressures at a total pressure of 1 atm. The data on NiO activities and directions of conjugation lines between coexisting oxide and spinel phases were used to establish the activity–composition relations in spinel solid solutions at 1300° and 1400°C. The MnAl₂O₄–NiAl₂O₄ solid solutions exhibit considerable negative deviations from ideality at these temperatures. The free energy of formation of MnAl₂O₄ from its oxide components (MnO + Al₂O₃) at 1300° and 1400°C is calculated to be −24.97 and −26.56 kJ. mol⁻¹, respectively. The activities determined in the stoichiometric spinel solid solutions are more negative as compared with those predicted from cation distribution models.     

Formation of ZnAl2O4 in the Presence of Cl2

The formation of ZnAl₂O₄ spinel in diffusion couples of Al₂O₃ and ZnO was investigated between 1000° and 1390°C in air and in air containing 4.8 vol% Cl₂ by X-ray diffraction, electron probe microanalysis, and scanning electron microscopy. The rate of formation of a spinel layer obeyed a parabolic rate law and was accelerated remarkably by the presence of Cl₂. The interdiffusion coefficient, , and the activation energy, E, were calculated to be 10⁻⁸ to 10⁻⁹ cm²/s and 123 kcal/mol (514 kJ/mol) in air and 10⁻⁷ cm²/s and 31 kcal/mol (130 kJ/mol) in air containing 4.8 vol% Cl₂, respectively.     

Preparation of NiAl2O4/SiO2 and Co2+-Doped NiAl2O4/SiO2 Nanocomposites by the Sol–Gel Route

NiAl₂O₄/SiO₂ and Co²⁺-doped NiAl₂O₄/SiO₂ nanocomposite materials of compositions 5% NiO – 6% Al₂O₃– 89% SiO₂ and 0.2% CoO – 4.8% NiO – 6% Al₂O₃– 89% SiO₂, respectively, were prepared by a sol–gel process. NiAl₂O₄ and cobalt-doped NiAl₂O₄ nanocrystals were grown in a SiO₂ amorphous matrix at around 1073 K by heating the dried gels from 333 to 1173 K at the rate of 1 K/min. The formations of NiAl₂O₄ and cobalt-doped NiAl₂O₄ nanocrystals in SiO₂ amorphous matrix were confirmed through X-ray powder diffraction, Fourier transform infrared spectroscopy, differential scanning calorimeter, transmission electron microscopy (TEM), and optical absorption spectroscopy techniques. The TEM images revealed the uniform distribution of NiAl₂O₄ and cobalt-doped NiAl₂O₄ nanocrystals in the amorphous SiO₂ matrix and the size was found to be ∼5–8 nm.     

Thermochemistry of MgAl2O4-Al8/3O4 Defect Spinels

Solution calorimetry of MgAl₂O₄-Al_8/3O₄ solid solutions was performed in a molten 2PbO · B₂O₃ solvent at 975 K. The results indicate small negative heats of mixing, relative to spinel standard states for both end-members. These data were combined with information on the energetics of the α-γ transition in Al₂O₃ and on the MgAl₂O₄-Al_8/3O₄ (MgO-Al₂O₃) subsolidus phase relations to estimate the partial molar entropy of mixing of γ-Al_8/3O₄ in the solid solution. This entropy is much less positive than that calculated from several models for the configurational entropy of mixing of magnesium, aluminum, and vacancies on octahedral and/or tetrahedral sites. The data suggest a good deal of local order to be present in the solid solutions, consistent with negative enthalpies of mixing and entropies of mixing far less than ideal configurational values.     

Thermodynamics of CuAlO2 and CuAl2O4 and Phase Equilibria in the System Cu2O-CuO-Al2O3

The standard Gibbs free energies of formation of CuAlO₂ and CuAl₂O₄ were determined in the range 700° to 1100°C, using emf measurements on the galvanic cells (1) Pt,CuO +] Cu₂O/CaO-ZrO₂/O₂,Pt; (2) Pt,Cu +] CuAlO₂+] Al₂O₃/CaO-ZrO₂/ Cu +] Cu₂O,Pt; and (3) Pt,CuAl₂O₄+] CuAlO₂+]Al₂O₃/CaO-ZrO₂/O₂,Pt. The results are compared with published information on the stability of these compounds. The entropy of transformation of CuO from tenorite to the rock-salt structure is evaluated from the present results and from earlier studies on the entropy of formation of spinels from oxides of the rock-salt and corundum structures. The temperatures corresponding to 3-phase equilibria in the system Cu₂O-CuO-Al₂O₃ at specified O₂ pressures calculated from the present results are discussed in reference to available phase diagrams.     

Optical Spectra of Synthetic Spinels in the System MgAl2O4–MgCr2O4

Unpolarized optical spectra were measured in the wavelength range 322–1666 nm by the diffuse reflection technique from spinel powders synthesized in the system MgAl₂O₄–MgCr₂O₄. The spectra were interpreted by the crystal-field theory on the basis of trigonally distorted spinel octahedra with D_3d symmetry. For chromium-rich solid solutions, including the MgCr₂O₄ end-member, results after peak fittings showed octahedral D_3d local symmetry around Cr³⁺ ions, identical to the crystallographic site symmetry. For chromium-poor solid solutions, however, octahedral C_3v local symmetry was suggested around Cr³⁺ ions, different from the D_3d crystallographically expected.     

Solubility and Crystal Growth of ZnAl2,O4 and AI2O3 in Molten PbF2 Solutions

The solubility of ZnAl₂O₄ (gahnite spinel) and NZOI (corundum) in their respective molten PbF₂ solutions was determined by a sealed-tube quenching method between 900° and 1250°C. Differential thermal analysis was used below 900°C. The solubilities (crystal-constituent/ PbF₂ weight ratio) at 1200°C were 0.151 for A1₂O₃ and 0.108 for ZnAl₂O₄, whereas at 900°C they were 0.093 and 0.048, respectively. These results are compared with those of single-crystal growth experiments and the crystalline products are described.     

Single-Source Sol-Gel Synthesis of Nanocrystalline ZnAl2O4: Structural and Optical Properties

Nanometer-sized zinc aluminate (ZnAl₂O₄) particles were synthesized from heterometal alkoxides, [ZnAl₂(OR)₈], possessing an ideal cation stoichiometry for the ZnAl₂O₄ spinel. ZnAl₂O₄ is formed at 400°C, which is the lowest temperature reported for the formation of monophasic ZnAl₂O₄. ²⁷Al magic-angle spinning nuclear magnetic resonance spectroscopy revealed that ZnAl₂O₄ possesses an inverse structure at <900°C, while the normal spinel phase is observed at higher temperatures. The homogeneity of the in-depth composition and Zn:Al stoichiometry (1:2) was confirmed by electron spectroscopy for chemical analysis. Evaluation of the valence-band spectra of ZnAl₂O₄ and ZnS suggested that the hybridization of O 2 p and Zn 3 d orbitals is responsible for lowering the bandgap in the latter. The average crystallite size showed an exponential relationship to the calcination temperature (X-ray diffractometry and transmission electron microscopy data). The optical spectra of different spinel powders (average particle sizes, 20–250 nm) showed that the absorption edge exhibits a blue shift as particle size decreases.     

Cation Self-Diffusion in Polycrystalline Y2O3 and Er2O3

A tracer sectioning technique was used to measure cation self-diffusion coefficients in fully dense polycrystalline Y_aO₃ and Er₂O_s under oxidizing conditions. The results are described by the relations for Y₂O₃ (1400° to 1670°C), and for Er₂O₃ (1400° to 1700°C). The greater activation energy for erbium diffusion in erbia may be partly attributable to a mass effect.     

Cation Self-Diffusion in Cr2O3

Self-diffusion of ⁵¹Cr was measured both parallel to and perpendicular to the c axis in single crystals of Cr₂O₃ as a function of oxygen partial pressure at 1490° and 1570°C. The oxygen-partial-pressure dependence of the diffusivity indicates that cation self-diffusion occurs by a vacancy mechanism. The values of the self diffusion coefficients determined in this experiment are about 10⁴ times smaller than those previously reported in this temperature range .     

High-Temperature Cation Distributions in Fe3-FeAl2O4

Cation distribution at 1600 K in the system Fe₃O₄-FeA1₂O₄ was derived from Seebeck coefficient measurements assuming that the aluminum distribution behaved according to the thermodynamic model proposed by O'Neill and Navrotsky. Approximately 10% of the aluminum resides tetrahedrally. This system provides a good test of thermodynamic models.     

Electrical Properties and Cation Valencies in Mn3O4

The electrical conductivity and thermopower of Mn₃O₄ were measured in the temperature range 920° to 1530°C. Electrical conduction in cubic Mn₃O₄ is explained by the small polaron hopping of electron holes between Mn⁴⁺ and Mn³⁺ on octahedral sites. The concentrations of Mn⁴⁺ and Mn³⁺ are governed by the disproportionation equilibrium 2Mn _oct ³⁺⇄Mn _oct ⁴⁺+ Mn _oct ²⁺. This model also explains the electrical behavior of NiMn₂O₄ and CuMn₂O₄.     

Thermodynamic Properties of Fe3O4-FeV2O4 and Fe3O4-FeCr2O4 Spinel Solid Solutions

Solid solutions of Fe304-FeV204 and Fe304-FeCr204 were prepared and equilibrated with Pt under controlled streams of CO/CO, gas mixtures at 1673 K. The concentration of Fe in Pt was used to determine the activity of Fe304 in the solid solutions. The activity of the second component was calculated by Gibbshhem integration. From these data, the Gibbs energy of mixing was derived for both systems. The experimental results and theoretical values which are determined from calculated cation distribution compare favorably in the case of vanadite solid solutions but not in the case of chromite solid solutions. The difference is attributed to a heat term arising from lattice distortion due to cation size difference. The positive heat of mixing will give rise to a miscibility gap in the system Fe304-FeCr204 at lower temperatures.     

The Series MgCr2O4-MgFe2O4

The characteristics of spinels in the series MgCr₂O₄-MgFe₂O₄ were determined. The plot of cell size vs. molar composition is unusual in series showing complete solid solution because an unusually large deviation from Vegard's law was observed. This deviation is caused by changes in spinel structure with composition and temperature, and an equation was derived which applies a correction in terms of the degree of inversion. The effects of temperature on compositions high in MgFe₂O₄ include changes in density and refractive index. Solid solution of forsterite in MgCr₂O₄ decreases the cell size to 8.329 A but apparently is less than 1%. Changes in composition caused by vapor loss or by dissociation are small enough that this series is essentially binary below 1400° C.     

Uniformly Porous Al2O3/LaPO4 and Al2O3/CePO4 Composites with Narrow Pore-Size Distribution

Porous Al₂O₃/20 vol% LaPO₄ and Al₂O₃/20 vol% CePO₄ composites with very narrow pore-size distribution at around 200 nm have been successfully synthesized by reactive sintering at 1100°C for 2 h from RE₂(CO₃)₃· x H₂O (RE = La or Ce), Al(H₂PO₄)₃ and Al₂O₃ with LiF additive. Similar to the previously reported UPC-3Ds (uniformly porous composites with a three-dimensional network structure, e.g. CaZrO₃/MgO system), decomposed gases in the starting materials formed a homogeneous open porous structure with a porosity of ∼40%. X-ray diffraction, ³¹P magic-angle spinning nuclear magnetic resonance, scanning electron microscopy, and mercury porosimetry revealed the structure of the porous composites.     

Activity–Composition Relations in MnCr2O4–CoCr2O4 Solid Solutions and Stabilities of MnCr2O4 and CoCr2O4 at 1300°C

Phase equilibria in the system MnO–CoO–Cr₂O₃ were investigated at 1300°C under controlled oxygen partial pressures by using the gas equilibration technique. The CoO activities in various phase assemblages of the system were measured by determining the partial pressures of oxygen in the gas phase for coexistence with metallic cobalt. The activity data revealed that at 1300°C, MnO–CoO and MnCr₂O₄–CoCr₂O₄ solid solutions exhibit mild positive departures from ideal behavior. The activities in the stoichiometric spinel solutions were found to be in good agreement with those predicted from a model based on cation distribution equilibria. The standard free energy of formation of the compound CoCr₂O₄ from its oxide components at 1300°C was determined as −37 636 J/mol, while that for MnCr₂O₄ was found as −44 316 J/mol.